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The Element
Hydrogen
Hydrogen (Latin: hydrogenium) is the
chemical element in the
periodic table that has
the symbol H and atomic number 1. At standard
temperature and pressure it is a colorless, odorless, non-metallic,
univalent, highly flammable diatomic gas. Hydrogen is the lightest and
most abundant
element in the universe. It is present in water and in all organic
compounds and living organisms. Hydrogen is able to react chemically
with most other elements. Stars in their main sequence
are overwhelmingly composed of hydrogen in its plasma state. This element is used
in ammonia
production, as a lifting gas, an alternative fuel, and more recently
as a power-source of fuel cells.
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| General |
| Name, Symbol,
Number |
Hydrogen, H, 1 |
| Chemical series |
nonmetals |
| Group, Period, Block |
1 (IA), 1 , s |
| Density, Hardness |
0.0899 kg/m3,
NA |
| Appearance |
colorless
|
| Atomic properties |
| Atomic weight |
1.00794 amu |
| Atomic radius (calc) |
25 (53) pm |
| Covalent radius |
37 pm |
| van der Waals radius |
120 pm |
| Electron configuration |
1s1 |
| e- 's per energy level |
1 |
| Oxidation states
(Oxide) |
1 (amphoteric) |
| Crystal structure |
hexagonal |
| Physical properties |
| State of matter |
gas |
| Melting point |
14.025 K ("434 °F) |
| Boiling point |
20.268 K ("423 °F) |
| Molar volume |
11.42 ×10-6
m3/mol |
| Heat of vaporization |
0.44936 kJ/mol |
| Heat of fusion |
0.05868 kJ/mol |
| Vapor pressure |
209 Pa
at 23 K |
| Speed of sound |
1270 m/s at 298.15 K |
| Miscellaneous |
| Electronegativity |
2.2 (Pauling scale) |
| Specific heat
capacity |
14304 J/(kg*K) |
| Electrical conductivity |
__ 106/m ohm |
| Thermal conductivity |
0.1815 W/(m*K) |
| Ionization potential |
1312 kJ/mol |
| Most
stable isotopes |
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| SI units & STP
are used except where noted. |
In the laboratory, hydrogen is prepared by reaction of acids on metals such as zinc. For production in
large scale commercial bulk hydrogen is usually manufactured by
decomposing natural gas.
Electrolysis of water is
a simple although inefficient method. Scientists are now researching
new methods for hydrogen production. One of them involves use
of green algae. Another promising method
involves the conversion of biomass derivatives such as glucose or sorbitol, which can be done at
low temperatures through the use of a new catalyst.
Notable characteristics
Hydrogen is the lightest chemical element with its most common
isotope consisting
of just a single proton and electron. At standard
temperature and pressure conditions, hydrogen forms a diatomic
gas, H2, with a boiling point of only 20.27 K and a melting point of 14.02 K.
Under exceedingly high pressures, like those found at the center
of gas
giants, the molecules lose their identity and the hydrogen
becomes a liquid metal
(see metallic hydrogen).
Under the exceedingly low pressure conditions found in space,
hydrogen tends to exist as individual atoms, simply because there
is no way for them to combine; clouds of H2 form and
are associated with star formation.
This element plays a vital role in powering the universe through
the proton-proton reaction
and carbon-nitrogen cycle.
(These are nuclear fusion processes
that release huge amounts of energy through combining two hydrogen
atoms into one helium
atom.)
The hydrogen atom
A hydrogen atom is an
atom
of the element hydrogen. It is composed of a single
negatively charged electron, moving
around the positively charged proton which is the nucleus of the hydrogen
atom. The electron is bound to the proton by the Coulomb force.
Applications
Large quantities of hydrogen are needed industrially, notably
in the Haber
process for the production of ammonia, the hydrogenation of fats and oils, and the production of methanol. Hydrogen
is used in hydrodealkylation,
hydrodesulfurization,
and hydrocracking.
Other uses:
Hydrogen can be burned in internal combustion
engines, and a fleet of hydrogen-burning cars is maintained by Chrysler-BMW.
Hydrogen fuel cells are being looked
into as a way to provide potentially cheap, pollution-free power.
History
Hydrogen (French for water-maker,
from Greek hudôr, "water"
and gennen, "generate") was first recognized as a distinct
substance in 1776
by Henry Cavendish. Antoine Lavoisier gave
the element its name.
Occurrence
Hydrogen is the most abundant
element in the universe, making up 75% of normal matter by mass and over 90% by number of atoms. This element is
found in great abundance in stars and gas giant planets. Relative
to its great abundance elsewhere, hydrogen is very rare in the
earth's atmosphere (1
ppm by volume). The most
common source for this element on earth is water which is composed two parts
hydrogen to one part oxygen (H2O). Other
sources include most forms of organic matter (currently all known
life forms), coal,
fossil fuels
and natural gas. Methane (CH4), which
is a byproduct of organic decay, is an increasingly important
source of hydrogen.
Hydrogen can be prepared in several different ways: steam on heated carbon, hydrocarbon decomposition
with heat, reaction of a strong base in an aqueous solution with
aluminium, water
electrolysis, or displacement
from acids with certain metals.
Commercial bulk hydrogen is usually manufactured by decomposing
natural gas.
Compounds
The lightest of all gases, hydrogen combines with most other
elements to form compounds. Hydrogen has an electronegativity of
2.2, so it forms compounds where it is the more non-metallic and
where it is the more metallic element. The former are called hydrides, where hydrogen either
exists as H- ions or just as a solute within the other
element (as in palladium hydride).
The latter tend to be covalent, since the H+
ion would be a bare nucleus and so has a strong tendency to pull
electrons to itself. These both form acids. Thus even in an acidic solution one sees ions like
hydronium
(H3O+) as the protons latch on to something.
Hydrogen combines with oxygen to form water, H2O, and releases
a lot of energy in doing so, burning explosively in air. Deuterium
oxide, or D2O, is commonly referred to as heavy water. Hydrogen also
forms a vast array of compounds with carbon. Because of their association
with living things, these compounds are called organic compounds, and
the study of the properties of these compounds is called organic chemistry.
Forms
Under normal conditions hydrogen gas is a mix of two different
kinds of molecules which
differ from one another by the relative spin of the nuclei. These two forms
are known as ortho- and para-hydrogen (this is different from
isotopes, see below). In ortho-hydrogen
the nuclear spins are parallel (form a triplet), while in para
they are antiparallel (form a singlet). At standard
conditions hydrogen is composed of about 25% of the para form
and 75% of the ortho form (the so-called "normal" form). The equilibrium
ratio of these two forms depend on temperature but since the ortho
form has higher energy (is an excited
state), it cannot be stable in its pure form. In low temperatures
(around boiling point), the equilibrium state is comprised of
almost only para form.
The conversion process between the forms is slow and if hydrogen
is cooled down and condensed rapidly, it contains large quantities
of the ortho form. It is important in preparation and storage
of liquid hydrogen since the ortho-para conversion produces more
heat than the heat of its evaporation and a lot of hydrogen can
be lost by evaporation in this way during several days after liquifying.
Therefore, some catalysts of the ortho-para conversion
process are used during hydrogen cooling. The two forms have also
slightly different physical properties. For example, the melting
and boiling points of parahydrogen are about 0.1 K lower than
of the "normal" form.
Isotopes
The most common isotope of hydrogen is 1H. This stable
isotope has a nucleus consisting of a
single proton;
hence the descriptive, although rarely used, name protium for 1H.
The other stable isotope is deuterium, 2H, with
an extra neutron in the nucleus. Deuterium
comprises 0.0184-0.0082% of all hydrogen (IUPAC); ratios of deuterium to protium
are reported relative to the VSMOW
standard reference water.
The third hydrogen isotope is the radioactive tritium, 3H. The tritium
nucleus contains two neutrons in addition to the proton.
Hydrogen is the only element that has different names for its
isotopes. The symbols D and T (instead of 2H and 3H)
are sometimes used for deuterium and tritium, although this is
not officially sanctioned. (The symbol P is already in use for
phosphorus and is not available
for protium.)
Precautions
Hydrogen is a highly flammable gas burning at concentrations
as low as 4%. It also reacts violently with chlorine and fluorine. Hydrogen, when mixed
with oxygen and lit, burns with an explosion.
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